For a variety of applications, the railroad industry being typical, the art has exhibited a need for a fail-safe timer. In this context, such a timer is a three terminal device, accepting an input stimulus (usually a voltage change) on an input terminal, and outputting a signal at its output terminal after a delay. Preferably, the delay should be capable of being selected within some range of delays. Were this the only requirement, the solution to the problem would be trivial inasmuch as a simple RC circuit would supply the need. However, the required timer should be vital in that the delay provided must be at least as long as the selected delay, and, of course, the timer tolerance should be within some predetermined range. The difficulty with the simple RC circuit is that variations in supply voltage as well as aging effects will have the effect of changing the delay presented by the circuit and, if this delay decreases, the circuit is not considered vital or fail-safe. Kolkman, in U.S. Pat. No. 4,059,845, discloses an alleged fail-safe time delay circuit although the time delay can be shortened by 30% due to circuit failures. Still other types of time delay circuits charge a capacitor through a transistor, or other active element. The difficulty with this arrangement in a vital timer, is that changes in the device characteristics can vary the charging current and therefore the time delay. Such an arrangement would not be considered vital since it admits of the possibility of decreasing the time delay. Finally, the conventional vital timer is a motor driven device whose characteristics it is desirable to improve from the standpoint of cost, weight, maintenance and repeatability of timing periods.
It is therefore one object of the present invention to provide a vital timer. It is another object of the present invention to provide a timer capable of timing out selected periods, with characteristics such that the delay actually produced is at least as long as the desired delay. It is still another object of the present invention to provide a vital time delay circuit in which the time delay is produced by charging an RC circuit, but which includes a balanced amplifier such that changes in supply potential or changes in the relationship between different supply potentials will not result in reducing the delay time produced.
It is a further object of the invention to provide a vital time delay circuit which produces a bi-polar output signal and which further comprises a pair of threshold sensing devices in an output circuit which requires the thresholds to be exceeded alternately and in sequence before producing the desired output signal. It is yet another object of the invention to provide a vital timer with a vital output stage producing as an output signal a potential not anywhere otherwise available in the circuit.